1. Field of the Invention
This invention relates to processes and methods for forming injection and production wellbores into a subterranean mineral deposit for providing in-situ recovery from that formation.
2. Prior Art
Today, with an increasing scarcity of surface accessible mineral formations, more and more attention is being paid to in-situ mineral recovery techniques and processes. As for example, in the area of oil shale formation refining, there has been increasing interest in practicing in-situ recovery techniques involving the forming of spaced-apart wellbores into a formation. A material for burning, steam under pressure, or the like, is then introduced into the formation through an injection system of wellbores. The material separates certain of the hydrocarbon components of that formation that are then withdrawn through one or more production wellbores. Heretofore, such in-situ refining has involved mining into or exposing the formation and then forming, with appropriate spacing from one another, injection wellbores into the formation along with one or more production wellbores to extend an appropriate distance therein. This in-situ process has been favored where the formation is exposed or nearly exposed to the surface, minimizing the difficulties in mining into the formation to form the production and injection wellbores. Further, similar in-situ methods have been employed in minerals recovery wherein production and injection shafts or wellbores have been placed into a subterranean formation. Whereafter, acid or steam under pressure is introduced through the injection shaft or wellbore to produce a chemical reaction. The minerals separated by that reaction are then removed through the production shaft.
Heretofore, in-situ mineral recovery systems have generally been practiced on fairly accessible formations. Particularly, for an in-situ process to be commercially practical for hydrocarbon recovery from coalbeds, oil shale or oil sands, only seams or formations close to the surface have justified the costs of tunneling and/or boring when compared with the value of the final product produced from such refining.
Where production systems involving drilling rather than tunneling have heretofore been practiced for recovering hydrocarbons from an underground formation, such have generally been limited to oil and/or natural gas production. The present invention, unlike such other earlier systems, is directed towards a process for commercially recovering gases from burning of a coal seam or bed that tilts or dips from the horizontal away from the surface. Only recently have drilling equipment and procedures been developed that allow for exactly drilling each wellbore to a desired location in a formation so as to obtain a flow of products of combustion thereacross.
In the past, where attempts have been made to recover gases by burning a formation such, unlike the present invention, have involved slant drilling techniques where a drilling rig is located and relocated each time a production or injection wellbore is drilled so as to provide spacing therebetween. In consideration of the drilling distance required in a slant drilling operation where the hole is drilled a distance that is the square root of the squares of the horizontal and vertical distance to a point in the formation, directional drilling becomes impractical at a relatively small distance from the surface for a coal seam that dips away from the drilling platform.
An attempt was made that utilized slant drilling techniques to install separate production and injection wells into a dipping coal seam. This attempt was made by Gulf Research and Development Company under a U.S. Department of Energy (DOE) Contract No. DE-AC03-77ET13108, and was known as Rawlins Module No. 2. This project involved drilling a number of wellbores from spaced-apart surface locations into a burn module that was located near ground surface. Each production wellbore in this project was drilled at a slant angle from the horizontal to a point location in that coal seam module, whereafter the drill string was then bent to drill within that seam. This process required that each drilling operation progress from a separate surface location at a slant angle to intersect the coal seam. Each wellbore thereby involved a substantial drilling distance. Of course, the drilling distance to the coal seam will increase dramatically as the seam depth and distance from the drilling site increases. Accordingly, utilizing such slant drilling techniques to form a number of production and injection wellbores quickly becomes uneconomical at coal seam module depths beyond a few hundred feet from the surface, rendering such procedure impractical.
The present invention provides a process for recovering gas from the combustion of burn module of a coal seam that is located at a depth well below the ground surface. The present invention provides for directionally drilling injection and production wellbores so as to minimize the drilling distance from the surface to points within the burn module and further provides for performing this drilling from essentially a single surface location and utilizing a single drilling platform. The present invention provides a drilling process that can be practiced at minimum cost so as to render practical the in-situ recovery of even deep portions of a dipping coal seam.